Fabrication process for a self-aligned optical subassembly

Abstract: A method is disclosed for forming a self-aligned optical subassembly for supporting an optical fiber and associated optical component(s). In particular, sequential masking layer/silicon substrate etch operations are performed so as to etch, in series, the largest opening first and the narrowest opening last. A series of mask/silicon etch operations may be performed to sequentially provide openings in the silicon of the desired thicknesses. That is, the thickness of masking layer (30) is now related (inversely) to the desired final depth of each opening. In particular, the masking layer comprises the greatest thickness (t₂) over the area associated with first groove (14), followed by a thickness of t₁ <t₂ over the area associated with second groove (16), where the masking material has been completely removed from the area associated with the deepest opening (12). Advantageously, the above-described process ot tailoring the thickness of the masking layer with respect to the desired final depths of the features (i.e., opening (12) and groves (14,16)) eliminates the need to perform any further photolithography operations in the formation of the final fiber stub subassembly. A first silicon etch may now be performed to provide for opening (12) to comprise a predetermined depth of d₁, where the predetermined may be controlled by the material used to etch, as well as the time and temperature conditions. For example, the etch may be performed using ethylenediamine pyrochatecol (EDP) at a temperature of 115°C in a nitrogen atmosphere, where it is known that this particular material will etch silicon at a rate of 25µm/hour. Subsequent to this first EDP etch, a masking layer etch operation (using, for example a plasma or buffered oxide) is performed to blanket remove a predetermined thickness of masking layer (30) sufficient to expose suface (11) of substrate (10) in the area of opening (16). This etch thus also reduces the thickness of masking layer (30) covering area (14). A second EDP etch is then performed so as to provide an opening (16) with a predetermined depth of d₂. The second EDP etch will also further etch first opening (12), resulting in opening (12) having a depth of d₁+d₂. After the second EDP etch is completed, a final masking material etch is performed to remove an amount of masking layer (30) sufficient to expose surface (11) of substrate (10) in the area of opering (14). A third EDP etch is then performed so as to form opening (14) to comprise a third predetermined depth d₃.

Description: Background of the Invention Technical Field The present invention relates to a self-aligned optical subassembly and, more particularly, to a method of processing a silicon substrate to form a self-aligned optical subassembly. Description of the Prior Art An exemplary utilization of silicon in the formation of a subassembly for optoelectronic devices is disclosed in U.S. Patent 4,945,400 (Blonder et al.) issued July 31, 1990 and assigned to the assignee of record in this application. In general, Blonder et al. disclose a subassembly including a semiconductor (e.g., silicon) base and lid including a variety of etched features (e.g., grooves, cavities, alignment detents) and metallization patterns (e.g., contacts, reflectors) which enable the optoelectronic device to be reliably and inexpensively mounted on the base and coupled to a communicating optical fiber. In particular, Blonder et al. disclose an arrangement wherein the optoelectronic device (e.g., LED) is disposed within a cavity formed by a lid member and the communicating fiber is positioned along a groove formed in a base member. A reflective metallization is utilized to optically couple the device to the fiber. Therefore, positioning of the device over the reflector is the only active alignment step required to provide coupling. Any remaining alignments are accomplished utilizing fiducial features formed in the base and lid members. Although the Blonder et al. subassembly represents a significant advance in the field of silicon optical bench packaging, a need remains for providing an arrangement which requires no active alignments to be performed. The provision of a completely passive optical packaging arrangement being considered both more reliable and less expensive than vinually any alternative requiring active alignment between components. EP-A-0465230 shows a method of etching portions of a silicon substrate to various depths in order to accommodate optical fibers and optical components. EP-A-0331333 also shows a method for etching portions of a silicon substrate to different depths to accommodate an optical fiber which has a bare portion and a coated portion. In accordance with the present invention, there is provided a method as defined in claim 1. The present invention relates to further advances in the field of silicon optical bench technology and, more particularly, to a method of fabricating a self-aligned optical subassembly for capturing an end section of an optical fiber (referred to as a "fiber stub") and providing alignment of the optical fiber to an associated optical component (either an active or a passive component) as further defined by the appended claims. Brief Description of the Drawing FIG. 1 contains an isometric view of an exemplary fiber stub subassembly formed using the process of the present invention; FIG. 2 contains an isometric view of a finished exemplary fiber stub subassembly of the present invention, where the subassembly of FIG. 1 is cut along illustrated line 2-2 to form the finished subassembly of FIG. 2; FIG. 3 is a side view of the subassembly as shown in FIG. 1, taken along line 3-3 of FIG. 1; FIG. 4 is a top view of an exemplary mask pattern which may be used to form the fiber stub subassembly of the above views; FIGs. 5-9 illustrate a series of photolithography steps used to pattern a masking layer so as to form different thicknesses of masking material with respect to the different openings as shown on the mask pattern of FIG. 4; and FIG. 10-14 illustrate the series of sequential etch operations useful in the method of forming a self-aligned fiber stub subassembly in accordance with the teachings of the present invention. Detailed Description An exemplary silicon substrate 10 containing a number of etched openings formed in accordance with the present invention is illustrated in FIG. 1. As shown, a large pyramidal opening 12 is used to allow for the capture of an optical component (e..g, LED, PIN photodetector or spherical lens). A first, relatively narrow groove 14, formed to the right of opening 12, is used to support a stripped end section of an optical fiber. Lastly, a second groove 16 is formed in tandem with first groove 14 and is used to support a coated fiber. In order to complete the fabrication of the fiber stub assembly, substrate 10 of FIG. 1 must be cut along line 2-2 to form the piece part as illustrated in FIG. 2. The fiber stub subassembly of FIG. 2 may then be mated with another substrate which supports (in an aligned configuration) the optical component to be coupled to the optical fiber. A complete description of such an arrangement may be found in U.S. Patent 5,179,609, issued on January 12, 1993 and assigned to the assignee of the present application. Advantageously, a number of alignment fiducials, such as fiducials 20 and 22 shown in FIGs. 1 and 2, may also be formed on substrate 10 and then used to provide both alignment and mechanical attachment of the fiber stub subassembly to a cover plate substrate (not shown). As mentioned above, an important aspect of optical packaging is the ability to provide passive alignment between components, in this case, between the optical fiber and an associated optical component. Such alignment is indicated in FIG. 1, by reference to line 3-3. FIG. 3 contains a side view of the substrate, taken along the optical axis line 3-3, and indicates the placement of both an optical component 24 within opening 12 and the support of an optical fiber 26, including a bare end section 28, as supported by tandem grooves 14 and 16. As will be discussed in detail below, the silicon processing steps of the present invention allow for passive alignment between component 24 and fiber end section 28 to be achieved. FIGs. 5-9 illustrate a set of exemplary steps used to pattern a masking layer so as to form different thicknesses of masking material in the previously-determined etch locations. FIGs. 10-14 then illustrate the series of sequential etch operations used to form a complete optical subassembly. FIG. 4 contains a top view of an exemplary mask pattern useful in forming the fiber stub subassembly described above. As shown, the pattern includes a relatively large feature associated with large opening 12 (for placement of the optical device), and a pair of tandem openings 14 and 16 disposed adjacent to opening 12. Evident in this view are necked-down regions 27 and 29 which are advantageously used to control the etching of both opening 12 and groove 16 with respect to groove 14. Without these necked-down regions, the etching operation used to form opening 12 may encroach upon the left hand side of groove 14, reducing its overall length. The same effect may result in over-etching groove 16 with respect to the right hand side of groove 14. Regions 27 and 29, therefore, will virtually eliminate this over-etching and result in forming groove 14 with the necessary length. FIG. 5 illustrates an exemplary silicon substrate 10 at the initial stage of the fabrication process. In particular, top surface 11 of substrate 12 is covered with a masking layer 30, where layer 30 may comprise any well-known masking material, including silicon dioxide. Layer 30 should be relatively thick, for example 2µm, but its exact thickness is not of great concern. As will be discussed in detail below, layer 30 is patterned to result in comprising a different thickness in areas which are designated to be etched to different depths. The difference in masking layer thickness, in accordance with the teachings of the present invention, may then be used to control the sequential etching operations used to form the tandem openings while maintaining optical alignment. Referring back to FIG. 5, masking layer 30 is subsequently covered by a photoresist layer 32 which is patterned so as to provide for the removal of photoresist in the area where openings 12,14 and 16 will be formed. The structure as shown in FIG. 5 is subsequently etched (using any suitable etching technique, such as plasma-assisted or wet chemical) until approximately two-thirds of masking layer 30 (in the exposed area to form openings 12, 14 and 16) is removed. The etch thus results in forming the structure as illustrated in FIG. 6. Referring now to FIG. 7, photoresist layer 32 is removed and a new photoresist layer 34 is deposited and patterned so as to expose only the area where opening 12 (the largest of the openings) is to be formed. The portion of masking layer 30 exposed as shown in subsequently etched (as before, any suitable etch may be used, for example, plasma-assisted or wet chemical) so as to remove approximately half of the masking material and thus form the structure as illustrated in FIG. 8. Subsequent to this etch step, photoresist layer 34 is removed and a new photoresist layer 36 is deposited and patterned as shown in FIG. 9 so as to expose the areas where openings 12 and 16 will be formed (the larger two openings when compared with opening 14). The structure as illustrated in FIG. 9 is then etched until surface 11 of substrate 10 is exposed in the area of opening 12. That is, the remaining portion of masking layer 30 associated with opening 12 is completely removed. The removal of the portion of masking layer 30 associated with opening 12 thus results in forming a reduced thickness of masking material 30 associated with the area of opening 16. The structure at this point in the process is illustrated in FIG. 10. In accordance with the teachings of the present invention, a series of mask/silicon etch operations may now be performed to sequentially provide openings in the silicon of the desired thicknesses. That is, the thickness of masking layer 30 is now related (inversely) to the desired final depth of each opening. In particular, the masking layer comprises the greatest thickness (t 2 ) over the area associated with first groove 14, followed by a thickness of t 1 < t 2 ) over the area associated with second groove 16, where the masking material has been completely removed from the area associated with the deepest opening 12. Advantageously, the above-described process of tailoring the thickness of the masking layer with respect to the desired final depths of the features (i.e., opening 12 and grooves 14,16) eliminates the need to perform any further photolithography operations in the formation of the final fiber stub subassembly. Referring to FIG. 11, a first silicon etch may now be performed to provide for opening 12 to comprise a predetermined depth of d 1 , where the predetermined may be controlled by the material used to etch, as well as the time and temperature conditions. For example, the etch may be performed using ethylenediamine pyrochatecol (EDP) at a temperature of 115°C in a nitrogen atmosphere, where it is known that this particular material will etch silicon at a rate of 25µm/hour. The structure at the end of this first silicon etch step is illustrated in FIG. 11. Subsequent to this first EDP etch, a masking layer etch operation (using, for example a plasma or buffered oxide) is performed to blanket remove a predetermined thickness of masking layer 30 sufficient to expose surface 11 of substrate 10 in the area of opening 16, as shown in FIG. 12. This etch thus also reduces the thickness of masking layer 30 covering area 14. A second EDP etch is then performed so as to provide an opening 16 with a predetermined depth of d 2 . The second EDP etch will also further etch first opening 12, resulting in opening 12 having a depth of d 1 + d 2 . FIG. 13 illustrates the structure at this point in the process. After the second EDP etch is completed, a final masking material etch is performed to remove an amount of masking layer 30 sufficient to expose surface 11 of substrate 10 in the area of opening 14. A third EDP etch is then performed so as to form opening 14 to comprise a third predetermined depth d 3 , as shown in FIG. 14. Further referring to FIG. 14, which illustrates the final structure of the fiber stub subassembly, opening 16 will comprise a final depth of d 2 + d 3 , and opening 12 will comprise a final depth of d 1 + d 2 + d 3 . Therefore, by knowing the desired final depths for all three openings, the intermediate depths required for the first EDP etch (to form a depth d 1 ) and the second EDP etch (to form a depth d 2 ) may be determined. Various other etched openings, for example, fiducials 20 and 22, may be formed during any desired EDP etch step as described above (or during subsequent etch operations), where the specific EDP step used (e.g., either first, second, third, or subsequent) will determine the final depth of the openings. In particular, if the masking layer is patterned and the process refined to include a fourth EDP etch. associated with the formation of fiducials, the fiducials will be formed to comprise a shallower depth than any of the previously-formed features. It is to be understood that the process as described above is exemplary only, and there exist many modifications, apparent to those skilled in the art, which are considered to fall within the scope of the present invention. For example, materials other than silicon dioxide may be used to form the masking layer (which in turn can serve as inter-metal dielectrics for multi-level metallization schemes for interconnections) and procedures other than the above-described EDP etch may be used to sequentially form the optically aligned openings, as described fully above.

Claims: A method of forming a fiber-supporting substrate (10) to provide for passive optical alignment between an optical fiber (26) and an associated optical component or components (24), the method comprising the steps of: a) providing a silicon substrate including a top major surface; b) forming a masking layer (30) on said silicon substrate top major surface; c) patterning said masking layer such that different areas of said masking layer comprise different thicknesses so that said thickness is inversely related to the desired final depth of each opening (12,14,16) to be etched in said substrate, said areas associated with the location of said optical fiber and said associated optical component or components; d) etching said masking layer until a first underlying surface portion of said silicon substrate exposed: e) etching said exposed silicon substrate to a predetermined depth; f) etching said masking layer again until a second underlying surface portion of the silicon substrate is also exposed; g) etching the exposed first and second portions of the silicon substrate to predetermined depths; and h) repeating steps (f) and (g) if necessary to expose and etch additional underlying surface portions until all openings required to support said optical fiber and said associated optical component or components have been formed; said steps (d) - (h) being performed without any additional masks formed over the areas of said masking layer. The method according to claim 1 wherein the optical fiber includes a bare end section (28) and a coated following section, and the method comprises: a) etching said exposed masking layer so as to remove essentially all masking material in the region where said associated optical component will be located, said etching step thus resulting in said masking layer having a first thickness t 1 in the area where said coated following section of said fiber will be located and a second thickness t 2 in the area where said bare end section of said fiber will be located, wherein the areas are formed to be in axial optical alignment; b) etching said exposed silicon to a first depth d 1 ; c) etching said masking layer for a period of time sufficient to expose the silicon surface in the area associated with said coated fiber section; d) etching said exposed silicon for a period of time sufficient to form an opening with a predetermined second depth d 2 in the area where said coated fiber will be supported; e) etching said masking layer for a period of time sufficient to expose the silicon surface in the area associated with the bare fiber end section; and f) etching said exposed silicon for a period of time sufficient to form an opening with a predetermined third depth d 3 in the area where said bare fiber end section will be supported. The method according to either claim 1 wherein in performing step b), a silicon dioxide layer is used as the masking layer. The method according to claim 3 wherein the silicon dioxide layer is a grown, thermal oxide. The method according to claim 3 wherein in performing the steps associated with etching the masking layer, a plasma etch operation is used. The method according to claim 1 wherein in performing the steps associated with the etching of silicon, ethylendiamine pyrochatecol, EDP, is used to perform the etching. The method according to claim 6 wherein in performing the etching with EDP, the etch is performed at a temperature of approximately 115°C in a nitrogen atmosphere.

Inventors: DAUTARTAS MINDAUGAS FERNAND, WONG YIU-HUEN, DAUTARTAS, MINDAUGAS FERNAND, WONG, YIU-HUEN
Applicants: AT & T CORP, AT&T CORP.
Application year: 1994
Main classification: H01L21
IPC codes: G02B6, H01S5, H01L21